The present invention relates generally to an organic electroluminescent (EL) array exposure head and an imaging system incorporating the same, and more particularly to an organic EL array exposure head adapted to make efficient use of light emitted out of each device of an organic EL array and an imaging system using the same.
The present invention is also concerned with an array-form exposure head fabrication process, especially a process for the fabrication of an organic EL array exposure head, etc. used on an imaging system.
So far, various organic EL array arrangements have been proposed for exposure heads for imaging systems. Pertinent prior arts are given below.
Patent publication 1 proposes that an organic EL array prepared on a glass or other insulating substrate in a batch fashion is combined with a separate driver IC wherein the images of light emitters of the organic EL array are formed on a photosensitive drum by means of a collective rod lens array.
Patent publication 2 proposes the use of a one-chip organic EL array comprising a plurality of rows but discloses nothing about the optical system for forming the images of light emitters on a photosensitive drum. The publication discloses that the EL layer in the organic EL array is deposited by evaporation.
Patent publication 3 teaches that microlenses are formed on the upper surface of a substrate by means of ion exchange or microlenses are prepared by a method wherein a photoresist is applied on the back surface of a substrate or a replica method and an organic EL array comprising resonator structures in alignment with such microlenses is deposited by evaporation.
Patent publication 4 relates to a process for the fabrication of an active matrix type organic display, showing that an organic light-emitting layer is formed on a glass substrate having a thin-film transistor by means of ink jetting.
Patent publication 5 teaches that the hole-injecting layer and organic light-emitting layer of an organic EL device are formed with a bank by means of ink jetting.
Patent publication 6 proposes a printer comprising a photosensitive drum in which a light-emitting layer is provided along with a TFT layer for control of light emission thereof.
Various proposals have also been made with a view to using other arrays such as LED arrays or liquid crystal shutter arrays as exposure heads for imaging systems. In most cases, reliance is on a collective rod lens array for collecting a light beam from the emitter of an LED array or the shutter of a liquid crystal shutter array onto a photosensitive drum.
Patent Publication 1                JP-A 10-55890        
Patent Publication 2                JP-A 11-198433        
Patent Publication 3                JP-A 2000-77188        
Patent Publication 4                JP-A 10-12377        
Patent Publication 5                JP-A 2000-323276        
Patent Publication 6                JP-A 2001-18441        
Patent Publication 7                JP-A 2000-353594        
Non-Patent Publication 1                “Polymer Type Organic EL Display”, The 8th Electronic Display Forum (Apr. 18, 2001)        
Non-Patent Publication 2                “Fine Imaging and Hardcopy”, edited by the Photography Society of Japan, the Joint Publication Committee of the Image Society of Japan, Corona Publisher Co., Ltd., Jan. 7, 1999, page 43        
In the above prior art wherein an organic EL array is used on the exposure head of an electrophotographic type printer, the use of a collective rod lens array for collecting a light beam from the emitter of an organic EL device in the organic EL array onto a photosensitive drum incurs an increase in the length of an optical path, resulting in an increase of the size of the arrangement. The use of the collective rod lens, because of being not located in one-to-one relation to each emitter, gives rise a periodical variation in the quantity of light. In addition, the collective rod lens is unavoidably expensive because high techniques are needed for its fabrication. A problem with microlenses is that crosstalk is likely to occur as a result of light that emerges from a specific emitter but is incident on an unassociated pixel position via a microlens adjacent thereto, not via the associated microlens, leading to a resolving power drop.
In an arrangement wherein substrates are applied to individual microlenses one by one, some problems arise with the mechanical strength required for maintenance operations such as that for removal for toner deposited to an optical system.
On the other hand, high-refractive-index ball lenses used for reflective paints, etc. available for enhancing the visibility of traffic signs at nighttime are very expensive, and so are expected to provide an optical system at very low costs if they can be used for array-form exposure heads such as organic EL array heads.